Tessellation circuits and methods are known in graphics processing systems to divide primitives into subprimitives represented by subvertices. Generally, primitive tessellation can be done in three ways; these are known as discrete tessellation, continuous tessellation, and adaptive tessellation. As known in the art, tessellation may be carried out to break down a primitive, such as a quad primitive which is defined by four vertices, triprimitives which are defined by three vertices, a line which may be defined by two vertices, or any other suitable primitive, into smaller subprimitives in an effort to get a more accurate depiction of an outer shape of a primitive, displacement mapping, or for any other suitable purpose. For discrete tessellation, an integer tessellation level per primitive is used. For example, if a line is a primitive, and the tessellation level is one, the line is divided into two equally spaced segments. When the tessellation level is two, the line is broken into three segments. The level is taken from the perspective of an edge of a primitive. Similarly, a triangle is subdivided into four sub-triangles for level one tessellation.
For continuous tessellation, floating point or fractional levels of tessellation per primitive are provided so that for example a tessellation level of 1.1 or 1.2 may be used to get a finer granularity of a breakdown of a primitive into smaller subprimitives. Adaptive tessellation also employs a fractional level of tessellation for each edge of a primitive and typically includes the application of continuous tessellation to an inner portion of the primitive and a seaming tessellation at the edges of primitives. With adaptive tessellation, the process typically includes performing continuous tessellation by tessellating uniformly, a primitive shape within the primitive being tessellated and applying an adaptive tessellation technique to an area outside the uniformly tessellated area.
Known tessellation engines are typically limited in their operation. For example, a typical tessellation engine may only accommodate one type of primitive such as a triangle primitive. In addition, tessellation engines may also typically carry out only one type of tessellation such as discrete or continuous tessellation. However, software based adaptive tessellation techniques are also known wherein a host processor may carry out an adaptive tessellation operation per primitive. This is typically performed since implementation of a tessellation engine in hardware can require large amounts of memory but memory is typically limited in graphics processors or other devices that carry out some type of tessellation. The software adaptive tessellation approach may also carry out another type of tessellation namely continuous tessellation. However such known techniques typically use a forward difference method which can result in holes between shared edges due to error that can be generated utilizing the forward difference method. This can result in discontinuous image generations. The algorithms to avoid holes between adjacent primitive edges tend to be complex and costly if implemented in hardware.
Since a host processor such as a CPU is required to carry out the software based adaptive tessellation using the forward difference method, lower precision can typically result and also decrease the CPU performance since it is required to carry out the tessellation operation to determine the tessellation vertices which may then be passed, for example, to a graphics processor.
Accordingly, a need exists for a tessellation circuit that does not require a host processor to carry out tessellation vertex generation but that would also accommodate at least all three types of tessellation. Alternatively it would be desirable to provide a tessellation circuit that provided both continuous and adaptive tessellation but that did not unnecessarily burden a host processor in the computations, while at the same time, minimizing the hardware cost.